As anyone who has read the railway stories of the Rev W Awdry will know, the real charm of the books lies in the illustrations rather than the words. The stories themselves are plain little things, with jerky sentences that are hard to read aloud with much parental conviction, and four basic plot lines: Troublesome Trucks, Proud Engine Gets His Comeuppance, Small Engine Shows His Worth, New Engine Is Shunned (And Then Wins Friends). The success of his 26-volume series (and the large profits that have come to the company that owns the television and image rights) owes much more to the artist who illustrated the first 11 stories: C Reginald Dalby, a Leicester man who was handily placed for Awdry's original publishers, Edmund Ward of Leicester.

When we think of, say, Thomas the Tank Engine, we think of Dalby's work: the bright blue boiler, the human face on the smokebox door, the eternal green fields of branch-line England. And if we think of Fox's Glacier Mints, which some us may do, we can also remember Dalby's work on the sweet wrappings, because Fox's was also a Leicester firm and Dalby drew its first trademark polar bear.

But what do Awdry's stories mean in the ever-growing world of literary and cultural theory? Until last week it wasn't a question that had announced itself to me. Then I was invited on to a radio programme which would discuss a new book about railways called Traintracks. As it happened, I couldn't do the programme but the BBC still kindly sent me proof sheets from the book. I noted the provenance of the two women authors - academia, a new university, social studies - and how often the word "discourse" occurred in the chapter heads. I skipped through the text, thinking not for the first time that the writing produced by social (or cultural, or media, or gender) studies is like seeing the world through half-closed Venetian blinds, or feeling it through rubber gloves, until my eye fell on a passage that is among the most thrilling interpretations of literature I have ever read.

The authors, Gail Leatherby and Gillian Reynolds, are keen to place Thomas, Gordon, James etc in their cultural context. It is "interesting" that Awdry should engender his engines as masculine. "Even more interestingly", the coaches are feminine: "Their location in the stories reflects dominant mid-20th-century masculinist attitudes to women. They are towed, pulled and 'looked after' ... they cry and sob when things go wrong ... True to the demands placed upon women in the 1950s, the gendered coaches pick up the emotional and unpaid labour of servicing capitalism (Hochschild 1983)."

I could go along with some of that, and even with the idea of troublesome trucks as representing Awdry's "perception of the working class". But then on the next page I came to Toby the Tram Engine, who, according to the authors, is the SS Empire Windrush of the series. The passage is worth quoting.

"It is acknowledged that many of the stories in the Railway Series are based on actual events. The story of Toby the Tram Engine first appeared in 1952, a time when thousands of potential workers were migrating from the West Indies to Britain. The Fat Controller met Toby whilst on holiday where he did not have enough work, and bought him. Toby is a different shape, a different colour (brown) and people fail to understand the way he operates. Toby's story is one of marginalisation and assimilation. He encountered prejudice but, after proving his usefulness, was partly painted blue, the same colour as Thomas. Thus, he was only partially drawn into the culture of Thomas and friends; his coach Henrietta, on the other hand, was painted brown all over [and] thus totally subsumed - rendered invisible - within the dominant feminine culture."

In our house, this passage has been subject to sustained examination. My 11-year-old son pointed out that Toby, while brown, has a white face (Reverse Al Jolson syndrome, Schumpeter 1986). I pointed out to him the tricky logic of the first two sentences. True, many of the stories were based on "actual events" (for example, the accident at the Salisbury curve when a locomotive collided with a luggage trolley and was - maybe - covered in marmalade). But whether West Indian migration is an example of Awdry deploying such an actual event is no more than unevidenced assertion.

Then, like any decent scholar, I went back to the original text. My copy of Toby the Tram Engine is a first edition, inscribed "From Gran and Granda 7th Feb [my birthday] 1953" and in reasonable condition apart from one or two drool marks, quite possibly from Fox's Glacier Mints. On the copyright page, there is an anouncement that the same author has written Our Child Begins to Pray ("A book for parents who wish for guidance in teaching their children how to pray" - I imagine a short print run, even in 1953). There follows a short introduction from "The Author" telling us that Toby is "a funny little engine with a queer shape" who works "very hard", and hoping that "you will like him too".

As for the story, the Fat Controller certainly meets Toby on his holiday, but this is in rural England rather than Jamaica and on an about-to-be-closed line (bus competition) where Toby takes "trucks from farms and factories to the main line, and the big engines take them to London". Henrietta's colour is a troubling question. While the Fat Controller certainly orders her to be painted brown, the fact is that she is brown already - evidence of some lack of communication between author and artist, which broke down completely four books later during work on Percy the Small Engine when Awdry gave Dalby the sack. (Dalby: "[He was] a pedantic, remote man with whom co-operation was difficult." Awdry: "To Dalby, one engine was very like another ... He could have ... seen real engines [any day], but he preferred to sit in his studio and draw what he thought was a good picture." Both quoted by Brian Sibley in Thomas the Tank Engine: The Complete Collection, 1996).

A little biographical research is also illuminating. When Awdry wrote about Toby, he was the vicar of Elsworth and Knapwell near Cambridge and thus close to the Wisbech tramway, to which he moved even closer when he changed parishes in 1953 and which I think (the materials are oddly not to hand) was one of the last lines in England to use steam tram engines. It seems unlikely that West Indian migration was much on his mind in rural East Anglia, but, yes, the subconscious is funny thing - though not quite as funny a thing as social studies.

Poor Wilbert Awdry, he knew not what he did. It may be as well that Leatherby and Reynolds leave off their study before we reach the later intrusion in the series of Donald and Douglas, two stubborn Scottish engines, painted black (Gordonbrownphobia, see Jack 2005).

· Ian Jack is editor of Granta magazine. Traintracks is published next week by Berg Publishers, £50 hardback and £16.99 paperback.